This invention relates to a laminated barrier film for packaging and methods therefor.
Polyolefinic materials such as low density polyethylene (LDPE), polypropylene (PP), polybutylene (PB), and polystyrene (PS), are used in high volumes by the packaging industry as one component of a laminate composite film for coating substrates such as paper, metal foils, nonwoven fabrics and the like. Such polyolefinic materials have substantially non-polar characteristics and thus are not typically well suited for adhesion to polar materials used to form some laminate composite films. Methods for increasing the adhesion between a polar and a non-polar film of a composite construction include the use of tie-layers having characteristics compatible with both the polar and non-polar films, and increasing the surface energy of one or both of the films by corona discharge methods and/or flame treatment methods. All of these prior methods require complicated fabrication techniques, thus the formation of such composite materials has heretofore been limited to particular manufacturing facilities.
When the non-polar polyolefinic layer is combined with a more polar polymeric layer such as ethylene vinyl alcohol copolymer, glycol modified polyethylene terephthalate, or polycarbonate, the laminate composite material thus formed is an effective barrier film. Such barrier films are useful for coating paperboard food containers to reduce the oxidation of perishable substances such as fruit juice, milk, and the like within the containers.
The laminate composite coatings useful as barrier films are typically extruded onto the substrate to be coated. The extrusion of non-polar polyolefin and more polar polymeric layers onto paperboard to produce barrier packaging materials is well known. See for example, U.S. Pat. Nos. 4,698,246; 4,701,360; 4,789,575; 4,806,399; 4,888,222; and 5,002,833.
In the production of laminated barrier films, the corona discharge method for obtaining adhesion between the non-polar and more polar layers produces a metastable oxidized high energy surface on the surface of the non-polar film. Such corona discharge high energy surface reverts back to a lower energy level after about 3 to 4 weeks. Hence, the shelf-life of the non-polar layer is relatively short requiring that the extrusion facilities be located in proximity to the barrier film facilities.
When a tie-layer is used to increase the adhesion between the more polar and the non-polar layers of a barrier film it is typically co-extruded with one or both of the films. The so called "tie-layer" is generally a chemically modified polymer which is compatible with the more polar and the non-polar layers and which can provide a suitable interface between the two dissimilar layers. In order to extrude a barrier film containing the tie-layer material, it is necessary, however, to utilize an additional extruder barrel, and a more complicated die feed-block arrangement. As a result of the added expense of such multiple extruders and complicated feed-block arrangements, prior construction of barrier films was limited to packaging facilities having the required co-extrusion equipment.
It is therefore an object of this invention to provide a laminated barrier film for packaging.
Another object of this invention is to provide a laminated barrier film formed from at least one non-polar first layer and at least one more polar second layer.
A further object of this invention is to provide a laminated barrier film formed from a non-polar first layer and a more polar second layer in the absence of a tie-layer.
It is yet another object of this invention to provide a method for forming a laminated barrier film.
A still further object of this invention is to provide a method for forming a laminated barrier film which method does not require the use of an additional extruder barrel and/or a complicated feed-block arrangement.
Other objects and benefits of the invention will be evident from the following discussion and appended claims.